Process for production of metals in an electric furnace

ABSTRACT

In the production of metals and metal alloys from a charge of material containing at least one of manganese, silicon, and chromium, the charge of material is disposed within a closed combustion chamber and combustion air is directed into the closed space in a volume based on the conditions existing therein. A sufficient amount of combustion air is supplied to completely burn all of the CO in the reduction gases to form CO2. The amount of combustion air supplied to the closed combustion space is in excess of the amount required to effect the complete combustion of the CO2. Further, during the reduction operation, the charge is worked by stirring means or the like to assure the permeability of its surface for the release of the reduction gases.

[54] PROCESS FOR PRODUCTION OF METALS IN AN United States Patent [111 3,615,346

[72] Inventors Johannes AQReth; I [56] References Cited Winiried H. Fettwels, both of Duisburg, UNITED STATES PATENTS Gummy 2 694 097 11/1954 Collin 13/2 X $55 111 1968 2,794,843 6/1957 Sem etal. 12/9x 3,303,257 2 1967 F t l. 139

[45] Patented Oct. 26, 1971 ujlwarae a l [73] Assignee Demag Eleltlrometallurgie G.m.b.II. pr'mary L'walton Duisburg, Germany Attorney-McGlew and Toren [32] Priority Feb. 19, 1968 [33] Gummy ABSTRACT: 1n the production of metals and metal alloys [31] D,556ls from a charge of material containing at least one of manganese, silicon, and chromium, the charge of material is disposed within a closed combustion chamber and combustion air is directed into the closed space in a volume based on the 323 conditions existing therein. A sufficient amount of combustion m g air is supplied to completely burn all of the CO in the reduc- [52] U.S.Cl 75/10, tion gases to form C0,. The amount of combustion air sup- 13/9,75/l1 plied to the closed combustion space is in excess of the [51] Int. Cl C22d 7/00, amount required to effect the complete combustion of the C2lc 5/52 CO Further, during the reduction operation, the charge is 50] Field of Search 13/l, 2, 9; worked by stirring means or the like to assure the permeability 75/10, l1, 12, 51; 266/30, 31; 9X, 2X, 1X ofits surface for the release ofthe reduction gases.

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SHEET U UF 4 Inventors :oMNNEfi R ETH wm mm: rsrrwus Armin/5Y5 PROCESS FOR PRODUCTION OF METALS IN AN ELECTRIC FURNACE SUMMARY OF THE INVENTION The invention relates to a process for the production of metals or metal alloys in an electric reduction fumace using a charge containing manganese and/or silicon and/or chromium.

While fully enclosed furnaces are in use for the production of pig iron, ferromanganese and calcium carbide, with the reduction gases being exhausted and cleaned, to date alloys containing silicon and chromium have been produced in open electric reduction furnaces.

Open furnaces are used in the production of the alloys specified above since relatively high temperatures-in the order of about 1,700 to 2,000 C.-are required for the production of alloys containing silicon and chromium, the exact reaction temperature being governed by the silicon content. At such high reaction temperatures, the reduction gases leave the surface of the charge at comparatively high temperatures; this can easily lead to the formation of sinter arches on the charge surface. These sinter arches or bridges greatly restrict the permeability of the chargesurface for gases; as a result, the sudden eruptions of very hot gases through the surface are encountered; the temperature of such gases can exceed 2,000 C. The eruption of hot gases greatly stresses all of the furnace elements and has very adverse effects on the furnace process. To avoid such eruptions, the charge surface must be worked at regular intervals. Normally, thick iron rods are used for this purpose; they are handled manually, or in the case of larger capacity furnaces, manually controlled poking or stirring machines mounted on the furnace platform are used.

In the case of open furnaces, the reduction gas generated in the reduction area is burned as it exitsfrom the charge surface. The reduction gas generated during such processes has a CO content of about 80 to 90 percent. For the complete combustion of one unit of this reduction gas a volume of air equaling roughly 2.5 times the gas volume is required.

This ratio is known as the air factor and equals n=l.

During the combustion of the reduction gas, temperatures in the order of 2,400' C. to 2,600 C. are generated. Normally, open furnaces are equipped with gas-collecting hoods connected to a stack for the evacuation of the waste gases formed. The waste gases are evacuated through the stack either by natural draught or aided by an exhaust fan, and then discharged into the atmospher Because of the high combustion temperatures and the stack draught exhaust process, such a volume of air is sucked into the combustion area between the surface of the charge and the gas hood that in practical operations waste gas temperatures of about 100 to 200C. are met.

This operation gives rise to relatively large volumes of waste gases. The table given below shows several examples for various processes and furnace sizes:

Generated Waste gas reduction volume at Furnace gases, 150 C size, Mw. Product NmJ/h. Nm.'/h factor, n

75% FsSi-.. 4,000 315,000 30 20 8101'( ,8 3,600 284,000 30 20 st-mafi 3,300 ass, 000 so 40 75% FeSl 8,000 630,000 30 40 SiCr (42% Bl). 7, 67, 000 30 example, 45 percent FeSi, SiCr and SiMn. In the case of a 20 MW furnace for the production of 75 percent FeSi, some 800 kg. of dust per hour or approximately 19,000 kgs. of dust per day are discharged into the atmosphere together with the waste gases.

Such dust is superfine and is composed mainly of SiO, and/or MnO and/or CrO, or combinations of these metals oxides, the composition depending on the process used The main reason for the generation of the dust can be traced to the fact that the metals reduced during the actual reduction process-in particular Si and Mn or Cr-are evaporated, and oxidize again on leaving the reaction chamber. These superfine particles are carried off by the gases leaving the furnace. In addition, fines from thefurnace charge are also removed by the exhaust gases.

For some considerable time, efforts have been made to remove the dust from the gases leaving open reduction furnaces to eliminate or at least reduce air pollution.

The investigations conducted thus far,,and the dust separation processes developed as a result of these investigations, have revealed that it is very difficult to remove the entrained dust in a gas-cleaning or dust separating plant, and that the specified degrees of purity of the cleaned waste gases can be achieved only with a substantial expenditure. Because of the relatively large volume of waste gases generated and the degrees of purity required for the gases, the plant necessary to remove the dust is of such a size that, compared with the furnace size, it is extremely uneconomical The same is also true for the investment, operating, and maintenance costs.

ln view of these factors, the object of the present invention is to change the mode of operation and design of open electric reduction furnaces so that the waste gases from such furnaces can be cleaned to the specified degrees of purity on an economically feasible basis.

ln accordance with the invention, this aim is achieved by reducing the charge in a furnace closed by a roof. In addition, combustion air is introduced into the furnace through intake openings having a variable inlet cross section. The volume of the combustion air supplied per unit of time is based on a continuously measured temperature and/or analysis of the waste gases exhausted from the furnace whereby the CO in the reduction gases is burned to CO, at a volume ratio of n=l .2 to n=5 and at a waste gas temperature of between l,000 and 1,500 C. measured at the point the gases leave the furnace chamber. The term n" denotes the ratio of combustion air to reduction gas under conditions of complete combustion, with the ratio of combustion air to reduction gas averaging 2.5:! and being variable within certain limits as a factor of the reduction gas analysis.

An advantageous feature of the proposed process provides for local working of the charge to ensure, at all times, that the surface layer remains uniformly loose, such working of the surface of the charge being effected by using one or more poking or stirring devices capable of continuous operation. The extent of the surface working performed is a function of the temperature distribution throughout the entire furnace chamber, the temperature being measured continuously be means of thermometer probes spaced over the area concerned and located in the vicinity of the furnace roof.

A further feature of the proposed process lies in the fact that the operation of the stirring devices for the continuous maintenance of a uniformly loose charge surface is manually controlled on the basis of visual observation of differences in brightness and the distribution of the same over the charge surface using one or more observation instruments that respond to heat radiation. Another advantage exists in the fact that such manual control of the stirring device movements is limited by a fixed control program that prevents the poker as such from being moved into a protective zone around the electrodes.

The further features of the invention are of particular advantage from the process technology aspect; in the main, these features provided for the device effecting a series of stirring operations in a limited section of the furnace in accordance with a fixed program and in such a way that certain areas lying on concentric circles are worked in sequence, and also for the fixed stirring program being capable of operation within a where n, air factor using open electric reduction furnaces-on limited section of the furnace-it being possible to select such 5 average 30--and section-as a function of the temperature gradients n; air factor using a furnace in accordance with the invenestablished by the continuous measurement of the temperation-approx. 1.2 to 5- ture distribution throughout the entire furnace chamber. 1.19

Finally, the proposed process is complemented and per- Example: =-m fected in that, as a function of the temperature distribution 10 throughout the entire furnace chamber as established by the This means that the waste gas volume is cut to approximatearea-spaced thermometer probes, one or more stirring devices y gutnth- H V I M can be employed in the limited furnace section exhibiting the The following table shows a comparison between the waste highest temperature gradients, and in that one or more obsergas volume of known open furnaces and those of furnaces in vation instruments responding to heat radiation can then be accordance with the invention:

Waste gas Waste gas Air volume volumes factor Generated of open using 1t Furnace reduction furnaces, Air new using ze, gas, 150/ 0., factor, grocess, new Mw. Product Nmfi/h. NmJ/h. n mJ/h process 75% F081 4,000 315,000 31,800 2 SICr 42% SI) 3, 000 30 2a, 700 2 Si-meta1. 3,300 257,000 30 26,000 2 40... 75% FeSI 8,000 030,000 30 63,600 2 40.-. SiCr (42% SI) 7, 200 670, 000 30 67, 400 2 set to the same limited section of the furnace, and also that the movement of the stirring device can be directed into the point exhibiting the highest temperature as a function of the highest local temperature established by the observation instrument.

It goes without saying that due attention has also been given to the furnace required for the process, which normally features a water-cooled roof. As usual, the furnace roof also features gastight apertures for the gas extraction hoods, charging tubes, and the electrodes.

In accordance with the invention, the main feature of the furnace lies in the fact that the roof of the furnace contains apertures arranged about its circumference to permit the entry of the air volume needed for the C0 combustion process, the effective intake cross section of such apertures being variable as a function of the furnace process data.

A further characteristic of the reduction furnace required to carry out the process is that the furnace roof or those parts of which that are directed toward the furnace shell and sealed off against the roof as such can be raised and lowered during furnace operation, their lifting movement being controlled as a function of the furnace process data.

In accordance with a further feature of the invention, the volume of air needed for combustion of the CO gas enters the furnace through one or several apertures and is controlled, by varying the suction rate of the gas exhaust facility, in the range of n=1 .2 to n=5 as called for by the furnace process condition.

Finally, one of the last features of the furnace roof design in accordance with the invention provides for several or all of the apertures in the furnace roof used to suck in air also being used as entry ports for the stirring or poking device arms.

The features of the invention permit a drastic reduction in the volume of waste gas encountered during the operation of electric reduction furnaces, while the expenditure for the overall furnace plant remains within economically justifiable limits and affords adequate consideration for the legal provi- 'sions regarding air pollution.

However, the advantages of the proposed invention are not exhaustively listed in the foregoing; in addition, all reduction processes that to date have been carried out in open electric reduction furnaces can, in principle, be effected in accordance with the invention if the combustion chamber is appropriately designed and the air inlet openings are properly controlled and distributed. Further, the invention also permits the air factor of approximately n=30 in the case of open furnaces to be reduced to between n=l.2 to n=5. This reduces the waste gas volumes of such open furnaces to the ratio of:

In addition, known types of enclosed electric reduction furnaces can be provided with apertures spaced about the circumference of the roof, which-as mentioned above-can be opened and closed. By opening a flap on such furnaces, the process can be regulated at any time during the operation of the furnace, whereas to date such furnaces always had to be switched off for such purposes. However, in the case of such an operation, the air factor is less than one.

The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this specification. For a better understanding of the invention, its operating advantages and specific objects attained by its use, reference should be had to the accompanying drawings and descriptive matter in which there are illustrated and described preferred embodiments of the invention.

DESQQIPTION OF THE DRAWINGS FIG. 1 is a vertical sectional view of a schematic representation of an enclosed electric reduction furnace embodying the present invention;

FIG. 2 is a plan view taken along the line A-B in FIG. 1;

FIG. 3 is a partial sectional view on an enlarged scale of the furnace roof shown in FIG. I in which an arm of a stirring device extends through an aperture;

FIG. 4 is a partial sectional view of the furnace roof similar to FIG. 3, but the aperture for combustion air intake is almost fully closed;

FIG. 5 is a horizontal section through the furnace roof along the line C-D of FIG. 1, with the positions of the stirring arm in the vicinity of an electrode indicated schematically;

FIG. 6 is a partial side view of the stirring device illustrated schematically, and indicating the various possible positions of the arm;

FIG. 7 is a sectional view through the top of the furnace, roughly corresponding to line C-D in FIG. 1, schematically indicating the stirring device and the area around an electrode that can be reached by its arm, and

FIG. 8 is a side view illustrating the stirring device shown in FIG. 7 with the various possible vertical positions of its arm.

A known open-type furnace l is shown with a roof 2 similar to the known enclosed furnace, which includes a known forced flow water-cooling system using tubular coils (not shown). The height of furnace roof 2 depends on the corn bustion or gas space between surface of the charge and the roof as required for the process in question.

The furnace roof 2 can be designed in the known way as a self-supporting structure positioned on the furnace shell 1a or on the furnace operating platform 3 and for sealing against the furnace shell in or for suspension from the next higher platform by using a suitable system. Electrodes 4, 4a, and 4b extend through the furnace roof in the known manner, each passing through an aperture cylinder 5 and being gastight. Placement of the charge into the furnace 1 is also effected in a known way using appropriately distributed charging tubes 7 extending through gastight seals in the furnace roof? The furnace roof 2 contains apertures 2a uniformly distributed about its circumference, or an annular gap may be used, the intake cross sections of which can be varied by means of electropneumatic and/or hydraulic and/or pneumatic control systems 8. Apertures 2a in furnace roof 2 are used for the intake of air (FIG. 4), needed for combustion of the reduction gases generated in the furnace 1.

Upon mixing with the air entering the furnace, the furnace reduction gas is fully burned in the combustion space between the furnace roof 2 and the surface of the charge 6. Complete combustion of the generated reduction gases calls for a volume of air equaling an average about 2.5 times the reduction gas volume. This air factor is normally expressed as: n=l.

In accordance with the invention, an air factor of between n=l.2 and n=5 is used for complete combustion and for cooling purposes during the operation of the furnace, the waste gases of the combustion process being exhausted from the furnace chamber by means of water-cooled exhaust ducts built into the furnace roof (not shown).

Furnace operation with an air factor of n=l.2 to n=5 produces approximate waste gas temperature of between l,500 and 700 C.

For the reasons stated above, the charge 6 must be worked for loosening its surface to maintain gas permeability in the case of processes involving high reaction temperatures. To this end, apertures 2a in the roof 2 of the furnace l are arranged so that, using a stirring or poking device 9 arranged to be moved around within the furnace 1, the surface of the charge can be uniformly worked across the entire furnace chamber, the device 9 moving on a circular track 3a arranged around the furnace l. The stirring device 9 works the surface of the charge through the air intake apertures 2a in a section of the furnace chamber that can be selected (FIGS. 5, 7), the working of the surface being carried out in accordance with a preselected program in such a way that points lying on concentric circles are worked in sequence, the programming unit leaving out a protective area around the electrodes 4, 4a and 4b.

Thermometer probes 2b are arranged over the area of the furnace roof 2 and continuously measure the temperature and indicate the temperature distribution over the entire furnace chamber. Selection of the furnace section to be worked by the stirring device is effected in accordance with the highest temperature gradient of the indicated temperature distribution.

The stirring operation in certain sections of the furnace can be initiated manually or automatically be integration of the temperature gradients. The temperature measured by thermometer probes 2b in furnace roof 2 and the temperature distribution in the furnace chamber permit determination of the section of the furnace in which the surface of the charge 6 must be worked. Accordingly, the stirring device 9 is moved into the appropriate position. Using observation instruments (not shown) that respond to heat radiation and show the temperature distribution across the surface of the charge on a viewing screen, an exactly positioned stirring operation can be initiated manually or automatically. Infrared cameras or similar appliances as used for military purposes can be employed, but these do not as such form part of the invention.

While specific embodiments of the invention have been shown and described in detail to illustrate the application of the inventive principles, it will be understood that the invention may be embodied otherwise without departing from such principles.

What is claimed is:

1. In a process for producing metals and metal alloys by reducing a charge, containing at least one material from the group consisting of manganese, silicon and chromium, in the furnace chamber of a closed electric reduction furnace, with the CO contained in the reduction gas being burnt in the furnace chamber with combustion air supplied to the furnace chamber and the waste gas products of the combustion being exhausted from the furnace chamber: the improvement comprising the steps of supplying combustion air to the furnace chamber at the minimum rate, coordinated with the chamber temperature, sufficient to maintain a volume ratio of n=1.2 to n=5, where n represents the ratio of combustion air to reduction gas sufficient to assure complete combustion and has an average value of 2.521 and is variable, within limits, in accordance with the analysis of the reduction gas; determining the nominal value of at least one parameter of the reduction process at which complete combustion of the reduction gas, with a minimum supply of combustion air, takes place; continuously checking such at least one parameter to determine variations from such nominal value; in accordance with determined variations from such nominal value, varying the rate of supply of combustion air to the chamber in a direction to restore such at least one parameter to its nominal value to maintain the value of n between 1.2 and 5; continuously measuring the chamber temperature at spaced points over the surface of the charge; and continuously maintaining a loose condition of the surface layer of the charge throughout substantially the entire chamber by effecting mechanical loosening of the surface layer at hot spots" as determined by such temperature measurements at spaced points over the surface of the charge.

2. A process, as claimed in claim 1, wherein the step of checking such at least one parameter comprises continuously measuring the temperature of the waste gases at their outlet from the furnace chamber.

3. A process, as claimed in claim 1, wherein the step of checking such at least one parameter comprises continuously analyzing the waste gases flowing out of the outlet from the furnace chamber to determine the percentage of complete combustion of the reduction gas.

4. A process, as claimed in claim 2, wherein the step of checking such at least one parameter comprises continuously analyzing the waste gases flowing out of the outlet from the furnace chamber to determine the percentage of complete combustion of the reduction gas.

5. A process, as claimed in claim 1, wherein the step of continuously measuring the chamber temperature at spaced points over the surface of the charge is effected by measuring the radiant heat at such points to determine differences in the distribution of radiant heatover the surface of the charge. A

6. A process, as claimed in claim 1, including the step of limiting the mechanical loosening to areas over the surface of the charge other than those areas immediately surrounding the furnace electrodes to avoid damage to the furnace electrodes. n

7. A process, as claimed in claim 1, in which such mechanical loosening is effected at successive points around the circumfe nce of circular areas ofthe charge surface.

8. A process, as claimed in claini l in which such mechani- V cal loosening is effected in accordance with the temperature gradient over the surface of the charge as determined by such temperature measure at spaced points over the surface of the charge. u g 1 9. A process, as claimed in claim 8, in which such mechanical loosening is effected at the furnace region having the maximum temperature in accordance with such temperature distribution; and further including the steps of concentrating the temperature measurements at such region and then effecting the mechanical loosening at those points within such region having the highest local temperatures. 

2. A process, as claimed in claim 1, wherein the step of checking such at least one parameter comprises continuously measuring the temperature of the waste gases at their outlet from the furnace chamber.
 3. A process, as claimed in claim 1, wherein the step of checking such at least one parameter comprises continuously analyzing the waste gases flowing out of the outlet from the furnace chamber to determine the percentage of complete combustion of the reduction gas.
 4. A process, as claimed in claim 2, wherein the step of checking such at least one parameter comprises continuously analyzing the waste gases flowing out of the outlet from the furnace chamber to determine the percentage of complete combustion of the reduction gas.
 5. A process, as claimed in claim 1, wherein the step of continuously measuring the chamber temperature at spaced points over the surface of the charge is effected by measuring the radiant heat at such points to determine differences in the distribution of radiant heat over the surface of the charge.
 6. A process, as claimed in claim 1, including the step of limiting the mechanical loosening to areas over the surface of the charge other than those areas immediately surrounding the furnace electrodes to avoid damage to the furnace electrodes.
 7. A process, as claimed in claim 1, in which such mechanical loosening is effected at successive points around the circumference of circular areas of the charge surface.
 8. A process, as claimed in claim 1, in which such mechanical loosening is effected in accordance with the temperature gradient over the surface of the charge as determined by such temperature measure at spaced points over the surface of the charge.
 9. A process, as claimed in claim 8, in which suCh mechanical loosening is effected at the furnace region having the maximum temperature in accordance with such temperature distribution; and further including the steps of concentrating the temperature measurements at such region and then effecting the mechanical loosening at those points within such region having the highest local temperatures. 